In motor protector devices, it is customary to use a dish-shaped composite thermostatic element to provide the actuation means for the device. The devices are typically embedded in the windings of a motor to provide inherent protection which senses not only overcurrent conditions but also overtemperature conditions. Both the amount of current flowing through the thermostatic element which provides self-heating and the ambient temperature determine if a fault condition occurs and consequently can cause the element to snap to an inverted dish-shaped configuration moving the contacts of the device apart breaking the circuit.
The temperature at which a composite snap-acting element actuates is determined by the material selection of the composite and the configuration of the dish-shaped deformation. The composite material will have a selected electrical resistivity which determines the heat generated in the element due to the electrical current passing through it and also a selected flexivity which is responsive to heat build up in the disc. This composite material with known properties is then deformed to a dish-shaped configuration, the extent of the dishing determining the temperature at which the disc will snap.
With new motor designs and better insulating materials used in them, higher motor operating temperatures are now achievable which require that motor protector devices also be designed to trip at higher temperatures than previously desired. However, it has been found that motor protectors provided for protecting motors at such higher operating temperatures have been less reliable than would be desired. It is also found that very high rejection rates are encountered in manufacturing motor protectors for such motors.
It has now been found that when snap-acting thermostatic elements are made for such higher temperature motors using conventional thermostat materials, the extent of deformation required to provide a desired high actuating temperature can be excessive and can exceed the failure limit of the material. In some cases where failure limit of the material is exceeded during deformation of the thermostat element, motor protectors incorporating the elements fail to operate at the desired temperature and have to be rejected. In other cases, where the deformation of the element material is less severe but still cause excessive permanent deformation of the material the snap-acting elements initially function at the proper operating temperature, however, these devices can quickly fail due to fatigue problems and lose their calibration. Also at high temperatures, calibration problems in the devices can occur because of the nonlinearity of the thermal response curve for conventional thermostat materials at higher temperatures.
Accordingly, it is an object of the present invention to provide an improved motor protector device for higher operating temperatures.
It is another object of the present invention to provide an improved multilayer composite thermostat material for use in motor protectors especially in high temperature applications.
It is yet another object of the present invention to provide an improved composite thermostat material for use as a snap-acting bimetallic disc which is durable in use, has good corrosion resistance properties and is relatively economical to fabricate.